Soybean Rust On The Move

Soybean growers in Australia, Asia, Africa and, most recently, South America have seen infected fields wither to a sickly yellow and suffer yield losses of up to 80%.

Those fields are all an ocean away. Yet U.S. producers realize it's only a matter of time — maybe mere months — before the soybean rust fungus infects and ravages American soybean crops.

“Soybean rust is inevitably going to show up in the U.S.,” says Monte Miles, USDA-Agriculture Research Service (ARS) plant pathologist. “We seem to have escaped the bullet for this past growing season, but I won't put any money on 2004.”

Bob Streit, an agronomist and crop consultant based in Iowa, agrees: “I would guess that sometime in the next two years we're going to see it.”

The soybean rust fungus produces spores carried by winds in the upper atmosphere — making it impossible to quarantine the disease. Worse yet, there are no known soybean varieties with genetic resistance to rust. And few fungicides are registered for use on soybeans to fight the disease.

Experts predict there are several ways spores could potentially be carried into this country. Besides via wind current from Africa or South America, those include: by tourists who have traveled internationally, via the movement of plant materials from infected countries, through the land bridge of Central America or even intentionally, through bioterrorism.

Miles says those are all possible scenarios, but believes the most likely way spores will infect the U.S. is through air movement, such as a hurricane. He has coined the phrase, “We are one good hurricane away from soybean rust.”

WHAT TO EXPECT

Asian soybean rust, scientifically known as Phakopsora pachyrhizi, was first reported in Japan in 1902. By the mid-1930s it was detected in Asia and Australia, and, by 1998, soybean fields in Africa had the disease. Soybean rust was reported in South America in 2000. This past season, the disease cost Brazilian farmers $1.3 billion in lost yield and chemical application.

The most common symptoms of the rust fungus are reddish-brown lesions on the underside of the soybean leaf. The lesions cause early leaf defoliation, resulting in fewer pods and seeds, and reduced yields.

Because the fungus is a subtropical disease, America's southern tier of states are particularly at risk.

Iowa State University Extension Plant Pathologist X.B. Yang is using computer modeling to simulate rust disease outbreaks on U.S. soybean-growing regions based on climate and wind patterns.

Yang says soybean rust is likely to show up in southern Florida or southern Texas first, because those are frost-free areas. The fungus could also initially survive in host plants in coastal areas such as Mississippi, Alabama or Louisiana during the warmer seasons.

Yang's computer simulations have predicted soy crop losses of 40-50% in the Gulf Coast region, where humid environmental conditions are right for the disease. He predicts yield losses of about 10% in the Midwest, where conditions aren't as favorable.

But given how the disease has spread so quickly over such a large acreage in South America, USDA-ARS researcher Reid Frederick says those estimates are likely “pretty conservative.” Frederick is one of the lead researchers on soybean rust at USDA's Biological Safety Level 3 Containment Facility in Fort Detrick, MD.

Additionally, Streit and fellow Iowa crop consultant Michael McNeill think that the Upper Midwest's longer hours of dew, coupled with cool nights, could make the disease act differently and severely threaten soybeans in Iowa, Minnesota, Illinois and Wisconsin. That's what happened with bean fields in the high plateau areas of the Brazilian Cerrado last season, they report.

PREPARING FOR COMBAT

Like other countries, the U.S. is scrambling to get a handle on the disease. Current research continues to focus on identifying effective fungicide application methods, as well as finding new compounds that can thwart the fungus. Researchers are also trying to identify germplasm that may offer genetic resistance to rust, but admit that a solution is years away.

In the short-term, a handful of fungicides are available to control rust. Some estimate it could take up to three applications per season, at a cost of about $15/acre per application, for the chemical alone.

“Because the fungicide applications could be so costly, some growers in marginal soybean production areas may be forced out of production … until genetically resistant varieties to rust are available,” says Frederick. “I wish we were 10 years further along with our research. Producers who have been affected are shocked at how quickly this thing moves through their fields.”

McNeill, of Algona, IA, has seen the disease in research settings and in fields in Brazil. “The key point for producers is to recognize what soybean rust is and take action immediately if it's found. You have hours rather than days to apply fungicides,” he says.

“Producers need to recognize this new disease. They also need to know what to do if it hits in order to be successful in getting their crops through to harvest,” he says. “I watched how producers reacted to soybean aphids this year, and many of them waited and took their time deciding if and when to spray. If you waited that long with soybean rust, you'd lose your crop.”